The present invention relates to a regulation system for providing electrical power. More particularly, the present invention relates to an alternating current ferroresonant transformer having a primary input winding, a secondary output winding, and a tertiary winding having a tank capacitor connected to terminals of the tertiary winding to form a resonant circuit.
Ferroresonant transformers are well known devices that maintain a substantially constant output voltage despite fluctuations of an input source voltage. Typically, the ferroresonant transformer includes a core and magnetic shunt assemblies forming closed loop paths and core sections for magnetic flux generated and/or linked by a primary winding, a secondary winding and a tertiary winding. A core section, which carries the flux, and which links the secondary winding, is generally heavily saturated. Output terminals of the secondary winding provide electrical power to a load, while a tank capacitor is connected across the terminals of the tertiary winding, and which may also be connected electrically to the secondary winding, to form a resonant circuit.
Ferroresonant transformers generally produce an output voltage signal which approximates an AC square wave due to the highly saturated secondary core. For DC applications, this square wave output need only be rectified to produce the desired regulated DC voltage source. For AC applications, either external or internal filters must be added to filter out the harmful higher harmonics of the square wave to produce an acceptable AC output voltage with an acceptable Total Harmonic Distortion (THD).
Although AC ferroresonant transformers and DC ferroresonant transformers include common elements described above, interconnection of the elements, placement of the elements on the core, and saturation of a portion or portions of the core are changed depending upon the type of output voltage signal desired. For example, a conventional DC ferroresonant transformer design usually entails a heavily saturated secondary core section in order to produce a regulated output voltage resulting from the fixed volt-time area of the magnetic core. As the saturation level of the secondary core section increases, the content of harmonic current produced at the output terminals of the secondary winding also increases. This type of ferroresonant transformer produces a square wave output voltage signal with high THD. In addition, the output terminals typically include a tap taken off the tertiary winding. This eliminates the need of having a separate secondary winding, but does require an increase in copper size of the tertiary winding to handle the vector sum of the tertiary and load current. Although a DC ferroresonant transformer does include magnetic shunt assemblies that form alternate magnetic paths within the core structure, typically, the magnetic shunts are lightly saturated or, perhaps, not saturated at all, in order to produce predictable results in the design.
Each of the aforementioned design approaches are used singularly or in combination and are appropriate in DC applications because harmonic content of the output voltage is not of concern. In fact, the resultant square wave is preferred because upon rectification and filtering, a constant DC voltage is realized.
In contrast to the DC ferroresonant transformer, an alternating current ferroresonant transformer must minimize harmonic content in the output power provided. A ferroresonant transformer of this type receives an input AC signal and produces a substantially constant sinusoidal output signal that has minimal harmonic content. Therefore, since the design goal for DC ferroresonant transformers is to increase harmonic content in the output signal, the teachings applicable to these types of ferroresonant transformers would not be applicable to AC ferroresonant transformers.
U.S. Pat. No. 2,694,177 describes an AC ferroresonant transformer to minimize harmonics in the output signal. The transformer includes a core formed of laminations having three distinct pairs of coil windows for windings. A primary winding and a compensating winding are disposed within the first set of coil windows. A secondary winding is disposed within the second set of coil windows, and a neutralizing (filtering) winding is disposed within the third set of coil windows with the secondary winding interposed between the primary/compensating windings and the neutralizing winding. Magnetic shunts are formed between each set of coil windows to form alternate magnetic paths in addition to a common magnetic path extending through each of the windings. A capacitor is connected in circuit with the secondary winding, the neutralizing winding and the compensating winding. An output signal is obtained across a terminal of the compensating winding and a node formed between the secondary winding and the neutralizing winding. This design is an example of a ferroresonant transformer producing an AC square wave and using an internal filter circuit to provide the desired AC sinusoidal wave with acceptable THD.
There is an ongoing need to provide a simpler alternating current ferroresonant transformer with less windings and/or less magnetic shunts in order to minimize construction costs, while further minimizing the harmonic content in the output signal and providing a substantially constant sinusoidal output signal for a given range of input voltages.